When a metal pipe or casing is fitted into a borehole, the metal will be liable to corrode as the fluids present in the borehole are potentially corrosive. Because an earth formation may include several different layers, large scale electrochemical cells can be set up between the parts of the casing contacting the different layers, so that in some regions, net current enters the casing from the formation while in other regions net current leaves the casing and flows into the formation. Where net current in the form of metal ions leaves the casing, the casing will corrode gradually. It is known that the rate of such corrosion can be in the order of mm/year, which is proportional to an outgoing current on the order of microamps/cm.sup.2. Since this electrochemical corrosion results from the current leaving the casing, the corrosion can be prevented if the casing is maintained at a potential such that net current enters the casing over its entire length. For this purpose, cathodic protection is often used. Cathodic protection is well known in the art, and prevents the electrochemical corrosion of a metal casing fitted in a borehole traversing an earth formation by making the metal casing into the cathode of an electrochemical cell.
In order to determine whether cathodic protection is needed for a casing, and in evaluating and/or optimizing the cathodic protection applied to a particular casing, the potential profile of the casing along its longitudinal direction is measured. The equipment for providing a typical potential profile for a casing is illustrated in FIG. 1. As shown, from ground surface 1 a borehole 2 traverses an earth formation which may include various different layers of different compositions. Fitted into the borehole 2 is a casing 3 which is typically comprised of a series of metal pipes connected in end-to-end relation. The equipment for measuring the potential profile of the casing typically comprises a downhole tool 4 which is suspended by a downhole cable 5 via a winch or pulley 6. The downhole tool 4 is provided with a pair of top and bottom electrodes 4a and 4b each of which is typically comprised of an electrically conductive roller so that the roller electrodes 4a and 4b may roll along the inner peripheral surface of the casing 3 as the downhole tool 4 moves up and down along the casing 3. Of course, the roller electrodes 4a and 4b are rotatably supported and electrically isolated from each other by a mass isolation joint 4c. The cable 5 includes at least two conductors, one for each of the roller electrodes 4a and 4b, and it is connected to a voltmeter 7. Accordingly, the potential difference between the roller electrodes 4a and 4b may be measured easily by the voltmeter 7 and the potential difference measurements may be carried out from point to point as the downhole tool is moved either upward or downward along the casing 3.
FIG. 2 illustrates a typical casing potential profile curve obtained by the equipment shown in FIG. 1. In FIG. 2, the ordinate represents the depth of the casing 3 from the ground surface 1 and the abscissa represents in microvolts the reading of the voltmeter 7. The depth of the downhole tool 4 from the ground surface 1 or top of the casing 3 is preferably determined to be the center point between the top and bottom electrodes 4a and 4b at the site of measurement. For the purpose of illustration, it is assumed that the solid line curve was obtained by running the downhole tool 4 along the casing 3 when the casing 3 was without cathodic protection. As indicated in FIG. 2, the solid line curve has four regions of interest. The section of the solid line curve indicated by I is a region having a negative voltmeter reading which is indicative of current flowing in a downward direction along the casing 3 in this region. On the other hand, the section of the curve indicated by II has a positive voltmeter reading and indicates the presence of upgoing current through the casing 3 in this region. Further, the curve includes a region III where the slope of the curve is negative, indicating the presence of current leaving the casing 3 radially, whereas, region IV of the curve has a positive slope which indicates that current enters the casing 3 in this region. As set forth previously, cathodic protection is generally needed if a region such as region III is present in the potential profile curve of a casing.
Where it is found that current is leaving the casing, some means such as seen in FIG. 3 must be provided to make the casing 3 entirely cathodic. In FIG. 3, a cathodic protection technique is used to cause the casing 3 to become a cathode with respect to the entire surroundings. Thus, a d.c. power supply 8 is provided with its negative polarity terminal connected to the casing 3 and its positive polarity terminal connected to an anode bed 9 embedded in the earth at a distance away from the casing 3. By providing the anode 9, a current flow from the anode 9 through the earth formation and into the casing 3 is produced, thereby counteracting or preventing a radially outward current flow from the casing. When the casing potential profile is measured under this cathodic condition, a curve as indicated by the dotted line in FIG. 2 is typically obtained. It should be appreciated that the microvolt value of the dotted line is always positive, thereby indicating the upward flow of current through the casing 3. Moreover, the slope of the dotted line is always positive, thereby indicating net current entering the casing 3 over its entire length. Thus, if the dotted line curve is obtained, one can be sure that the casing 3 is protected to some degree.
It is to be noted that a casing potential profile curve as shown by the dotted line in FIG. 2 is not always obtained. Often the casing potential curves show many changes in slope. These changes could either correspond to changes in the axial current resulting from current entering or leaving the casing radially, to the flow through the casing of an essentially constant axial current where the resistance of the casing is varying, or most likely to a combination of the two effects. One reason for the casing resistance to vary is the existance of electrochemical corrosion which is often concentrated locally. Thus, it is often desirable to find the location of severe corrosion and the rate of corrosion. Knowing the location and the rate, one can take various possible alternative measures which would maximize the usage of the casing 3. In order to evaluate the degree of cathodic protection or rate of corrosion, it is necessary to measure with high accuracy both the potential difference along the casing and the local casing resistance. After these parameters are measured, one can calculate the local current flowing along the casing, which, in turn, allows one to calculate the value of the radial current leaving the casing 3.
Some techniques for determining local currents along a casing have been proposed, such as are disclosed in U.S. Pat. No. 2,459,196, issued to W.H. Stewart on Jan. 18, 1949, and U.S. Pat. No. 4,431,964, issued to A. M. Walkow on Feb. 14, 1984. However, these prior art techniques are more or less insufficient in accuracy and are limited to operation in wells which contain insulating fluid such as diesel oil or gas. The requirement for having a nonconductive fluid in the casing in order to perform a corrosion evaluation is particularly limiting, because during repair, overhall, and maintenance operations when such an evaluation is often desired, wells often contain conductive fluids such as brine. The conductive fluid must then be displaced by diesel oil before the prior art techniques can be used. Thus, there has been a need for a new technique which can provide more accurate measurements and be operable in any well fluid, thereby saving time and expense involved in pretreating the well prior to investigation.